tony sayer wrote:

Accuracy.. over;( or under reading

OIC what you mean: do they read high or low? I don't know really.
You'd expect a rotating-disc meter to under-read at low power levels due
to the friction in the bearings, but they have some sort of compensation
for that, I think, with various tweaky-screws inside the meter. In
principle power factor shouldn't make a difference because the
accelerating torque on the disc at any moment is proportional to the
instantaneous v*i product. If that's so then over a whole number of
cycles the disc's movement will be proportional to the net energy flow -
IOW the meter should respond to the true mean power. Errors, I guess,
could then be in either direction. There are doubtless particular
trends for actual practical meter designs, but I wouldn't like to
speculate on that. Are there any meter specialists reading?

As to the electronic ones I have _absolutely_ no idea. How do they
work? Do they sample and digitise v and i first and then compute energy
flow in the digital domain - in which case accuracy will be down to the
performance of the CT and VT and the ADCs - or do they use an analogue
front-end with a 4-quadrant multiplier and integrator? - in which case
the scope for errors would be rather greater...

--
Andy